This invention relates to an electrically-powered drivetrain configuration for a vehicle, and more particularly to a multiple ratio transmission driven by a pair of electric motors.
In a series electric drivetrain, an electric motor is drivingly coupled to the vehicle wheels through a transmission gear arrangement, and electric power for operating the motor is obtained from an engine-driven generator, storage batteries, or a fuel cell, for example. To minimize the power requirements of the electric motor, the transmission may be designed to provide two or more speed ratios, much like a transmission for an internal combustion engine. In such case, shifting from one speed ratio to another can be performed either by using hydraulically-activated clutches to absorb the shift energy or by mechanically shifting the transmission during an input power interruption. The hydraulic approach significantly increases the cost of the drivetrain, while the mechanical approach requires an undesired power interruption. Accordingly, what is desired is a series drivetrain that has the cost advantages of the mechanical approach, without a power interruption.
The present invention is directed to an improved series electric drivetrain in which a multiple speed, mechanically-shifted transmission is driven by a pair of electric motors, and where the input power is provided by both electric motors during operation in any given speed ratio, and by only one of the electric motors during shifting between speed ratios. Preferably, the electric motors are each rated at one-half of the continuous input power requirement of the drivetrain, and one of the motors is released from the output during shifting while the other carries a peak load of twice the continuous rating. When the released motor has decelerated to the post-shift speed, a mechanical clutch is synchronously engaged to complete the shift, and the motors resume a shared supply of the transmission input power. In this way, the cost advantages of a mechanically-shifted transmission are realized without requiring a power interruption during transmission shifting.